Multi-planet systems compared
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The system of 55 Cancri has five detected planets that span a broad range of masses, from about 10 times Earth to 4 or 5 times Jupiter. All evidently travel on more or less circular orbits, like planets in the Solar System. The host is a Sun-like G8 star located at a distance of 12.53 parsecs (41 light years) in the constellation Cancer. It is cooler and less massive than our Sun, with a bolometric luminosity only 60% Solar (Fischer et al. 2008). The yellowish primary star has a small binary companion of spectral type M4 and mass 0.26 MSOL, orbiting at a semimajor axis of about 1000 AU (Desidera & Barbieri 2006). The wide separation between the two stars suggests that neither would substantially inhibit the evolution of planets around the other. In fact, the red dwarf companion, 55 Cancri B, is a potential exoplanet host in its own right. Future radial velocity searches will establish whether it harbors its own planetary system. Takeda and colleagues note that the mass and age of 55 Cancri are poorly constrained. This uncertainty seems to be a function of the star’s unusually high metallicity (0.315). Nevertheless, they are confident in assigning values very close to Solar for both parameters: a mass of 0.96 MSOL and an age of 5 billion years (Takeda et al. 2007). This estimate is supported by the star's leisurely rotation period of 39 days (Fischer et al. 2008). Thus the star's planetary companions may have reached an evolutionary stage similar to those in our own system, whose age is estimated at 4.6 billion years. system architecture Like the Solar System, the system of 55 Cancri has clearly demarcated inner and outer regions (see comparative diagram). The inner system comprises a cluster of four planets, all orbiting within a semimajor axis of 0.8 AU. This configuration is dominated by its most massive member, the Jupiter-size second planet (b), and it includes a Super Earth of about 10 MEA (e) and two additional objects that fall between the ice giants and the gas giants in mass (c, f). Then, after a gap of 5 AU, the outlying fifth planet (d) traces a wide orbit with a period of more than 14 Earth years, even longer than Jupiter's period of 12 years. This fifth planet is 55 Cancri's true Jupiter analog. Its minimum mass of almost 4 MJUP and its relatively circular orbit must have exercised strong constraints on the system’s evolution, just as Jupiter did in our system. The resulting configuration fulfills key predictions regarding the correlation of metallicity with planetary evolution (Greaves et al. 2007). Enhanced stellar metallicity is associated with giant planet formation as well as inward migration. In the case of 55 Cancri, four giant planets evolved, with three of them (planets b, c, and f) orbiting close to the central star and the fourth and most massive (d) remaining beyond the system's ice line. The three innermost planets bear a strong family resemblance to the system of Gliese 876. Each system contains a star-hugging ensemble of planets comprising two gas giants (or quasi-gas giants) orbiting in or near a mean motion resonance, plus a smaller planet on an interior orbit. In each case, the inner planet is a tidally circularized Super Earth that has evidently been shepherded into its present position by the inward migration of the two more massive planets (Ida & Lin 2005, Fogg & Nelson 2005, Mandell et al. 2007; see also Crowded Orbits). 55 Cancri resembles Gliese 876 in another important way, insofar as in both systems the orbital inclination of the most massive planet has been estimated by photometric observations (Rivera et al. 2005, Fischer et al. 2008). If we make the reasonable assumption that the remaining planets in each system have similar inclinations, then we can calculate the actual mass as well as the minimum mass for each one. In the case of 55 Cancri, the inclination is approximately 53 degrees. |
Crowded orbits
Index of exoplanetary topics
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five planets
migration and evolution The fifth planet has probably undergone little or no inward migration during its evolution, in contrast to the giant planets in the inner system. These planets evidently spiraled inward through the system's primordial gas disk to reach their present positions, in a process known as "Type II migration" (see Evolution of Planetary Systems). Gravitational perturbations forced rocky planetesimals into the shrinking area within the radius of the second planet’s orbit, where the planetesimals underwent collisions, accretions, and ejections. The result was the formation of the system's closest planet, the hot Super Earth that we now observe in its star-grazing orbit of less than three days (Fogg & Nelson 2005, Raymond et al. 2006b, Mandell et al. 2007). Last update July 2008 |
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All text is copyright Raymond Harris 2006-2008 |